(i) Field of the Invention
The present invention relates to a dipyrromethene metal chelate compound, and an optical recording medium in which said compound is used and which is recordable and reproducible at a higher density as compared with a conventional technique.
(ii) Description of the Related Art
As a recordable-type optical recording medium which conforms to compact disc (hereinafter abbreviated to "CD") standards, a CD-R (CD-Recordable) has been suggested and developed {e.g., Nikkei Electronics No. 465, p. 107 (Jan. 23, 1989), and OPTICAL DATA STORAGE DIGEST SERIES, Vol. 1, p. 45 (1989)}. As shown in FIG. 1, this CD-R comprises a transparent resin substrate 1, and a recording layer 2, a reflective layer 3 and a protective layer 4 laminated in this order on the transparent resin substrate 1, and when the recording layer is irradiated with a high power laser light, the recording layer gives rise to a physical or a chemical change to record information in the form of pits. The information in the form of the pits can be reproduced by irradiating a formed pit site with a low power laser light to detect the change of a reflectance. For the record and the reproduction of the optical recording medium, a near infrared semiconductor laser having a wavelength of 770 to 830 nm is usually used, and since the optical recording medium conforms to the standards of CDs such as Red Books and Orange Books, it has a feature that it is compatible with a CD player and a CD-ROM player.
However, a record capacity of the above-mentioned conventional medium is about 650 MB, and when the record of digital movies is taken into consideration, the capacity is insufficient. In recent years, with the remarkable increase of information volume, demands for the enhancement of the density and the capacity of the information recording medium have increased more and more.
Furthermore, the development of short wavelength semiconductor lasers which can be utilized in an optical disc system has advanced, and red semiconductor lasers having wavelengths of 680 nm, 650 nm and 635 nm have been put to practical use {e.g., Nikkei Electronics No. 592, p. 65 (Oct. 11, 1993)}. By shortening the wavelength of the record/reproduction laser and increasing a numerical aperture of an object lens, the size of beam spots can be reduced, which permits the formation of the high-density optical recording medium. In fact, the optical recording medium having the large capacity in which the digital movies can be recorded for a long time has been developed by shortening the wavelength of the semiconductor laser, increasing the numerical aperture of the object lens, or using a data compression technique {e.g., Nikkei Electronics No. 589, p. 55 (Aug. 30, 1993), and No. 594, p. 169 (Nov. 8, 1993)}. Nowadays, there has been developed a digital video disc (DVD) in which the digital movies are recorded for a period of 2 or more hours. The DVD is a read only medium having a record capacity of 4.7 GB, and it has been desired to develop a recordable optical disc suitable for this capacity.
In addition, a laser of 532 nm obtained by high-frequency conversion of a YAG laser has also been put to practical use.
A blue/green semiconductor laser of 490 nm which is far shorter than 532 nm has also be researched, but its practicable level has not been attained {e.g., Applied Physics Letter, p. 1272-1274, Vol. 59 (1991), and Nikkei Electronics No. 552, p. 90 (Apr. 27, 1992)}.
In the case that the short wavelength laser is used, a linear record density and a radial record density of the optical disc can theoretically equally be increased, but under the existing circumstances, it is difficult to increase the radial record density so as to be equal to the linear record density. Since the laser light is diffracted and scattered by grooves or lands, the narrower a track pitch is, the smaller a signal detection light quantity is. It is limited from the viewpoint of a molding technique to narrow the track pitch while maintaining a groove depth sufficient to obtain a tracking signal. Moreover, if the grooves are deep and narrow, it is difficult to form a uniform recording layer. In addition, edges of the grooves and the lands are not smooth but slightly rough, which causes noise. Such a bad influence noticeably occurs at a position where the track pitch is narrow to some extent. Taking these facts into consideration, it can be supposed that when the numerical aperture of the object lens at a wavelength of 520 nm is 0.6, the limit of the groove pitch is about 0.5 .mu.m.
When a dye layer of the CD-R medium is irradiated with the laser light to bring about a physical change or a chemical change and to thereby form the pits, an optical constant and a decomposition behavior of the dye are important factors for forming the good pits. If the less decomposable dye is used, sensitivity deteriorates, or if the noticeably decomposable or the easily changeable dye is used, the interrelation of the pits and the radial land portions are largely affected, so that the formation of the reliable pits is difficult. In the conventional CD-R medium, a refractive index of the dye layer is low at a laser wavelength which is used at a high record density, and an extinction coefficient is not a suitable value, so that the reflectance is low and a sufficient modulation amplitude cannot be obtained. In addition, the small pits should have been formed by a focused beam, but there are inconveniently formed the pits which are spread so largely as to have a large influence on an environment, and a radial cross talk deteriorates. On the contrary, in some cases, the pits are extremely small, so that the desired modulation amplitude cannot be obtained. Therefore, it is necessary to select the dye for the recording layer having the suitable optical characteristics and decomposition behavior.
For example, Japanese Patent Application Laid-open No. 199045/1994 has suggested an optical recording medium which is recordable and reproducible by a semiconductor laser having a wavelength of 680 nm. In a recording layer of this medium, a cyanine dye is used, and the probability of the high-density record and reproduction is exhibited, but there is not any description that the recording has actually been carried out at the high density.
Furthermore, in U.S. Pat. Nos. 4,774,339, 4,916,711, 5,248,782, 5,274,113 and 5,498,641, there have been disclosed chelate compounds of dipyrromethenes and boron halides, but a description of the optical recording media using these compounds is not present.